Like
most of the epidemiological studies we need to have control data to
determine what "unusual" meant. However, we canít do the comparison among
individuals since everyone is different. Rather, the comparison should be
done within the individual. In another words, we will compare the
individualís activity before the disease onset to his/her usual activity.
Thus here, we have a "case" component and also a "control" component, but
the information of both components will come from the same individual.